docs: def callback(status, message, job, result, exception, stacktrace):
      """Example callback function.
  
      :param status: Job status. Possible values are "invalid" (job could not be
          deserialized or was malformed), "failure" (job raised an exception),
          "timeout" (job timed out), or "success" (job finished successfully and
          returned a result).
      :type status: str
      :param message: Kafka message.
      :type message: kq.Message
      :param job: Job object, or None if **status** was "invalid".
      :type job: kq.Job
      :param result: Job result, or None if an exception was raised.
      :type result: object | None
      :param exception: Exception raised by job, or None if there was none.
      :type exception: Exception | None
      :param stacktrace: Exception traceback, or None if there was none.
      :type stacktrace: str | None
      """
      assert status in ['invalid', 'success', 'timeout', 'failure']
      assert isinstance(message, Message)
  
      if status == 'invalid':
          assert job is None
          assert result is None
          assert exception is None
          assert stacktrace is None
  
      if status == 'success':
          assert isinstance(job, Job)
          assert exception is None
          assert stacktrace is None
  
      elif status == 'timeout':
          assert isinstance(job, Job)
          assert result is None
          assert exception is None
          assert stacktrace is None
  
      elif status == 'failure':
          assert isinstance(job, Job)
          assert result is None
          assert exception is not None
          assert stacktrace is not None